1. Field of the Invention
The present invention relates to radiation systems.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is trarisferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Typically, lithography systems use lasers as radiation sources to produce an illumination beam, e.g., a coherent illumination beam or a partially coherent illumination beam. During its travel through the lithography system, the illumination beam may reflect from components in the lithography system, which can form scattered light. The scattered light can interfere with the illuminating beam causing speckle patterns in an image. The speckle patterns are undesirable because they can cause errors in a pattern formed on a substrate. The speckle patterns can be caused by interference of partially coherent beams that are subject to minute temporal and spatial fluctuations. The speckle patterns are sometimes referred to as noise-like characteristics of the (partial) coherent illumination beam. Speckle patterns can also be caused when an element that increases angular distribution is used because multiple coherent copies of the beam are made. The multiple coherent copies of the beam can interfere with each other when an optical path difference between the different coherent copies (e.g., between generation of the beams and detection of the beams) is small compared to a coherence length (e.g., transverse and temporal) of the beams.
Conventionally, the speckle patterns have been compensated for through use of a diffractive or refractive optical element positioned after the laser, which are used to form an incoherent beam from the coherent beam. These elements are sometimes referred to as “coherence busting elements.” As discussed above, the incoherent beam comprises multiple copies of the coherent beam.
The speckle pattern can be further reduced through movement of the optical element with respect to the illumination beam. The movement of the optical element changes a phase distribution for each copy of the coherent beams, which changes the speckle pattern for set of copies of the coherent beam. Through integrating (e.g., summing) of all the speckle patterns, uniform light is produced. However, a significant movement of the optical element is needed to substantially eliminate the speckle patterns. Also, typically the significant movement must be done within a short period of time, for example an exposure time. In an example where 30 pulses are used from a 1000 Hz laser, the exposure time could be about 30 μs. The significant movement in this short period of time can cause large oscillations within the lithography system, including high acceleration and jerks. The high acceleration and jerks can cause problems within the lithography system. Also, due to typically limited integration time, e.g., about 50 ns per pulse, it becomes nearly impossible to move the optical element enough with respect to the beam to substantially eliminate the speckle patterns.
Another way of compensating for the speckle patterns is to use a large number of laser pulses, e.g., 60 laser pulses, during each exposure cycle. A different speckle pattern results from each laser pulse. Thus, through use of a large number of laser pulses, the speckle patterns can be averaged out over time. However, recent lithography systems have decreased the number of laser pulses and/or have reduced the duration of each laser pulse during each exposure cycle. Unfortunately, reducing the number of laser pulses during each exposure cycle may not allow for the averaging effect to occur. Further, it may be difficult to move an optical element an acceptable amount during a reduced laser pulse duration to allow for compensation of the speckle patterns.
Therefore, what is needed is a system and method that produces incoherent radiation having uniform intensity.